A die bonder is an apparatus that picks a semiconductor chip or die from a loader and places it onto a carrier such as a lead frame. To ensure the quality and reliability of subsequent processes performed on the die, a bondhead of the die bonder should be aligned with respect to a supporting surface of the lead frame such that the die it picks up is plane-parallel to the supporting surface within several tenths of an arcsecond even when there is a load on the bondhead. Alignment is also important to reduce the risk of the fragile semiconductor die cracking. Therefore, the bondhead is designed to be adjustable to tilt with respect to the supporting surface in order to align it with the supporting surface.
Typically, there are two methods for aligning a bondhead with respect to a supporting surface. These are the contact and the non-contact methods. FIG. 1 illustrates a contact type alignment tool 100 that has a bondhead 12 with a circular-rimmed ruler 14 mounted at one end of the bondhead 12. The ruler 14 presses onto a piece of carbon paper that is placed onto a supporting surface 16. If the ruler 14 is misaligned with respect to the supporting surface 16, an incomplete circle will appear on the carbon paper. The bondhead 12 is then adjusted until a complete circle appears when the ruler's bottom surface is parallel to the supporting surface 16, which then indicates a proper alignment of the bondhead 12 with respect to the supporting surface 16. This circular ruler 14 simulates a collet of the bondhead impacting the supporting surface during pick and place operations on a die. However, the impact condition of the circular ruler 14 on the support surface deviates from the actual pick and place process. Furthermore, it is time consuming to adjust the tilting of the circular ruler 14 using an incomplete circle as a gauge.
FIG. 2 shows another contact type method of aligning the bondhead 12 with respect to the supporting surface 16 which is discussed in more detail in U.S. Pat. No. 7,066,373 entitled “Method For Aligning The Bondhead Of A Die Bonder”. A contact needle 18 is used to determine the actual tilt of a die 20 during the pick and place process. The die 20 is first picked up by the bondhead 12 before touching the needle 18. The corresponding vertical position of the bondhead 12 is read by the encoder as Z1. The vertical positions of at least two more points, Z2 and Z3, are measured to determine the tilt of the die 20 relative to the supporting surface 16 using a simple geometric relation. This method of alignment is simple and can align the bondhead with the die 20 to several tenths of a micron. However, the eccentric point loading on the die 20 cannot reflect impact conditions on the contact surface during actual pick and place operations on the die.
FIG. 3 illustrates a non-contact method of alignment which is also disclosed in U.S. Pat. No. 6,179,938 entitled “Method And Apparatus For Aligning The Bonding Head Of A Bonder, In Particular A Die Bonder”. In this method, a non-contact sensor 22 determines the tilting angle of the bondhead 12. The bondhead 12 picks up a ferromagnetic or reflective die 20 and places it on the non-contact sensor 22 such as a calibrated magnetic or optical measuring device. The accuracy of detection of the tilting angle depends on the type of sensor used. As this is a non-contact method, it cannot determine the surface loading effect when the bondhead 12 places the die 20 onto a surface, which potentially may cause the die 20 to crack.
The sensors 22 for the above described non-contact method can be in the form of force measurement sensors developed for measuring moments acting on the corresponding load surfaces, such as that disclosed in the U.S. Pat. No. 6,990,867 entitled “Force Detection Device”, wherein multiple capacitive displacement sensors are attached to the surfaces of two box-like structures to measure forces and moments. Although the invention disclosed in this prior art can measure up to three forces and three moment components, the square structure of the apparatus and the large number of sensors required render the apparatus complicated to use and to manufacture. It is therefore desirable to devise a simple, fast and accurate method of adjusting the alignment of the bondhead in relation to the supporting surface.